The field of the present invention relates to control of an internal combustion engine having a variable compression ratio, and in particular to fuel injection control.
Variable compression ratio (VCR) engines are equipped with various mechanisms to adjust the volumetric ratio between piston top dead center and piston bottom dead center. Such a VCR engine changes the compression depending on various operating conditions to provide improved performance.
However, the inventors herein have recognized disadvantages of such VCR engines. For example, changing compression ratio during engine operation may introduce an air-fuel ratio error. In particular, since changing compression ratio changes engine breathing characteristics, a change in inducted airflow can result in an air-fuel ratio error. This air-fuel ratio error can increase emissions.
The inventors have further recognized that conventional feedback air-fuel ratio control may not effectively minimize these air-fuel ratio errors under all operating conditions. In other words, simply relying on adjustments based on exhaust gas oxygen sensors may provide a degraded response in certain operating conditions.
Finally, the inventors have recognized that the air-fuel ratio error can be especially difficult to minimize when the engine is operated under open loop air-fuel ratio control, since no feedback mechanism is provided to compensate for the air-fuel ratio error.
Disadvantages of prior approaches are overcome by a method for operating an internal combustion engine, the engine having a variable compression ratio, the method comprising: determining a fuel injection amount based on a parameter indicative of a compression ratio of the variable compression ratio engine; and injecting fuel into the engine based on said fuel injection amount.
By taking into account variation in engine compression ratio, more accurate air-fuel ratio can be obtained. This can be especially true during transients of compression ratio. Such improved air-fuel ratio control can decrease emissions.
Note that there are various ways to calculate fuel injection amount based on compression ratio. For example, it can be done by adjusting engine breathing maps, or adjusting engine-operating parameters. Further; it can be done using manifold pressure sensor based fueling systems or mass airflow sensor based fueling systems. Various other embodiments are described later herein.
Also, note that there are various ways to inject fuel into the engine based on a fuel injection amount. For example, adjusting a fueling command signal, or changing a number of times fuel is injected, or changing fuel vapor introduced via an evaporative emissions system can affect injected fuel. Any such method can be used according to the present invention. Various other embodiments are described later herein.
Finally, note that there are various other features of the invention that can be performed in various ways. For example, any type of variable compression ratio can be used, such as one where connected rod length changes or where piston height changes.